The present invention relates to a heater-equipped oxygen sensing element which is preferably installed in an internal combustion engine to detect the concentration of oxygen gas involved in the exhaust gas and control an air-fuel ratio of the internal combustion engine.
To control the air-fuel ratio, internal combustion engines have oxygen sensing elements provided in their exhaust passages.
For example, a conventional oxygen sensing element comprises a cup-shaped solid electrolytic body having an inside space serving as a reference chamber, a sensing electrode provided on an outer surface of the solid electrolytic body so as to be exposed to measuring gas, and a reference electrode provided on an inner surface of the solid electrolytic body. The sensing electrode and the reference electrode may extend entirely or partly on the inner and outer surfaces of the solid electrolytic body (refer to Unexamined Japanese Patent Application No. SHO 58-73857).
Furthermore, an electric heater is disposed in the reference chamber. This kind of oxygen sensing elements do not operate properly until their temperature reaches a predetermined temperature level. Thus, the heater is usually equipped to quickly increase the temperature of the oxygen sensing element when the ambient temperature is low, thereby reducing a deactivated duration and correctly measuring the oxygen concentration.
However, this kind of conventional oxygen sensing elements have the following problems.
The outer surface of the oxygen sensing element has a gas receiving surface region extending from a distal end of the sensing element to a position spaced by a distance L away from the distal end of the sensing element. The gas receiving surface region is exposed to the measuring gas whose temperature increases to a higher temperature level during the operation of this sensing element.
When the sensing electrode and the reference electrode are formed entirely on the outside and inside surfaces of the solid electrolytic body, the oxygen sensing element produces a sensor signal equivalent to a composite output from a plurality of electric circuits sequentially arranged from a high-temperature section to a low-temperature section along an entire surface of the solid electrolytic body. When the oxygen sensing element has a low-temperature portion, its sensor output and response will deteriorate due to insufficient activation at the low-temperature portion.
Furthermore, similar problems may arise even when the sensing electrode and the reference electrode are formed partly on the outer and inner surfaces of the solid electrolytic body. For example, when these electrodes are located in the low-temperature region, the sensor will produce an inaccurate sensor output due to insufficient activation at the low-temperature portion.
The oxygen sensing elements, when installed in the exhaust passage of an internal combustion engine, need to produce an accurate sensor output within a short period of time after the internal combustion engine starts its operation. To satisfy this requirement, the oxygen sensing elements must operate properly with a short dead time which is required for the heater to increase the temperature of the solid electrolytic body to a predetermined active level. These requirements were difficult goals to attain for the conventional oxygen sensing elements.
In view of the conventional problems, the present invention has an object to provide an oxygen sensing element that is rapid in activation and excellent in response.
To accomplish this and other related objects, one aspect of the present invention provides an oxygen sensing element comprising a cup-shaped solid electrolytic body with one end closed and an inside space serving as a reference gas chamber, a sensing electrode provided on an outer surface of the solid electrolytic body so as to be exposed to measuring gas, a reference electrode provided on an inner surface of the solid electrolytic body, and a heater disposed in the inside space of the reference gas chamber. A contact portion comprises a region where the heater is brought into contact with the inner surface of the solid electrolytic body and an opposing region on the outer surface of the solid electrolytic body. The sensing electrode includes at least part of the contact portion. A gas receiving surface region, exposed to the measuring gas when the oxygen sensing element is operated, is provided on the outer surface of the oxygen sensing element so as to extend from a distal end of the oxygen sensing element to a position spaced by a distance L away from the distal end of the oxygen sensing element. At least part of the contact portion is located in a region extending from the distal end of the oxygen sensing element to a position spaced by a distance 0.4L away from the distal end of the oxygen sensing element. And, the sensing electrode is entirely located in a region extending from the distal end of the oxygen sensing element to a position spaced by a distance 0.8L away from the distal end of the oxygen sensing element.
With this arrangement, the inner surface is brought into contact with the heater at the contact portion. The sensing electrode includes at least part of the contact portion. FIG. 5 shows the contact portion including an inner point xe2x80x9cPixe2x80x9d where the heater is brought into contact with the inner surface of the solid electrolytic body and an outer point xe2x80x9cPoxe2x80x9d opposing the inner point xe2x80x9cPixe2x80x9d via the solid electrolytic body, together with a neighboring region including the vicinity of these points xe2x80x9cPixe2x80x9d and xe2x80x9cPo.xe2x80x9d
The contact portion on the inner surface may be a point (or points), a line (or lines), or a surface (or surfaces) which depends on the contact condition between the heater and the inner surface. The heater may be brought into contact with the inner surface at a single portion or a plurality of portions.
The gas receiving surface region is directly exposed to the measuring gas when the oxygen sensing element is operated. The gas receiving surface region has a neighbored surface region that is not exposed to the measuring gas. A clearance between the gas receiving surface region and the neighbored surface region is sealed by a packing, such as a metallic spring, which is capable of preventing gas leakage.
At least part of the contact portion is located in the region extending from the distal end of the oxygen sensing element to the position spaced by the distance 0.4L away from the distal end of the oxygen sensing element. If the contact portion is located at an altitudinal level higher than this region, heat leakage toward the upper low-temperature region of the oxygen sensing element will increase. This will result in insufficient temperature increase at the contact portion. Activation of the oxygen sensing element will be delayed.
The sensing electrode is entirely located in the region extending from the distal end of the oxygen sensing element to the position spaced by the distance 0.8L away from the distal end of the oxygen sensing element. If the sensing electrode is not completely located in this region, the sensing electrode temperature may decrease locally. Such a local temperature reduction will result in deteriorated response in the sensor performance.
The heater includes a resistor element generating heat in response to supplied electric power. It is preferable that the heat-generating resistor element opposes the measuring electrode. With this arrangement, it becomes possible to effectively heat the sensing electrode, enhancing the activity of the oxygen sensing element.
The oxygen sensor of the present invention operates in the following manner. The inner surface of the oxygen sensing element is brought into contact with the heater at the contact portion. The sensing electrode includes at least part of the contact portion.
Heat generated from the heater is directly transmitted to the sensing electrode via the inner surface and the solid electrolytic body. Thus, the sensing electrode is directly heated by the heater. Accordingly, the present invention reduces the activation time required from initiation of heating by the heater to generation of an accurate sensor signal from the activated sensing element.
The contact portion is located in the region extending from the distal end of the oxygen sensing element to the position spaced by the distance 0.4L away from the distal end of the oxygen sensing element. With this arrangement, it becomes possible to reduce the heat leakage toward the upper low-temperature region of the oxygen sensing element. Thus, the heating efficiency is improved.
The sensing electrode is entirely located in the region extending from the distal end of the oxygen sensing element to the position spaced by the distance 0.8L away from the distal end of the oxygen sensing element. Thus, the sensing electrode can maintain a high temperature during operation of the sensing element (refer to FIG. 8), realizing uniform temperature distribution and satisfactory response. The activation time is shortened.
The present invention provides an oxygen sensing element rapid in activation and excellent in response.
As the sensing electrode is partly provided on the solid electrolytic body, the total cost for the electrode is low compared with a case where the electrode is entirely formed on the surface of the solid electrolytic body.
The sensing electrode may be formed at the distal end of the sensing element, or may be formed along a cylindrical side portion of the sensor body other than the distal end as shown in FIG. 15.
It is preferable that the electrode area is larger than 2 mm2. If the electrode area is less than 2 mm2, an obtained sensor output will be insufficient.
According to the present invention, it is preferable that sensing electrode and the reference electrode are in a confronting relationship via the solid electrolytic body.
The sensor output is obtained when oxygen ion current flows between the sensing electrode and the reference electrode. From the functional view point, a portion of the electrode can be omitted if this portion is offset from an opponent electrode. Both the sensing electrode and the reference electrode are made of a noble metal or the like.
Accordingly, the present invention reduces the total amount of the electrode material and therefore reduced the total cost of the oxygen sensing element.
According to the present invention, it is preferable that an external lead electrode extends on the outer surface of the solid electrolytic body to transmit a sensing signal of the sensing electrode to the outside, and the external lead electrode has a circumferential width in a range from 0.1 mm to 5 mm.
The external lead electrode shrinks when it is exposed to the high-temperature gas. If the width of the external electrode is less than 0.1 mm, the external lead electrode may be broken due to the progress of shrinkage.
On the other hand, if the width of the external electrode exceeds 5.0 mm, the sensor output and response may deteriorate due to the influence of the low-temperature portion of the external lead electrode.
It is possible to provide a plurality of external lead electrodes, since the total number of the external lead electrodes is not limited to a specific number. In this case, it is preferable that the sum of the widths of the plurality of external lead electrodes is less than 5.0 mm.
The external lead electrode can be formed by plating, paste printing, sputtering, or evaporation.
According to the present invention, it is preferable that an internal lead electrode extends on the inner surface of the solid electrolytic body to transmit a reference signal of the reference electrode to the outside, and the internal lead electrode and the external lead electrode are in an offset relationship via the solid electrolytic body.
This offset arrangement is effective to reduce the oxygen ion current flowing between the internal lead electrode and the external lead electrode. Adverse influence given to the sensor output is reduced, and the response of the oxygen sensing element is improved.
According to the present invention, it is preferable that the sensing electrode is formed by chemical plating.
In general, chemical plating, conductive paste printing, sputtering or evaporation is preferably used in the formation of various electrodes.
The electrode formed by the chemical plating has excellent surface energy and catalytic activity because of a sintering temperature lower than that of the paste electrode. The response is higher.
Furthermore, compared with the electrode formed by sputtering or evaporation, the electrode formed by the chemical plating has numerous fine pores which contribute the diffusion of oxygen and therefore improve the response.
Prior to the chemical plating, a noble metallic nucleus of a predetermined pattern is provided on the outer surface of the solid electrolytic body. Subsequently, the chemical plating is performed on the solid electrolytic body to form an electrode having the same pattern as that of the noble metallic nucleus. Even a complicated sensing electrode can be easily formed.
The noble metallic nucleus is formed in the following manner.
An organo-metallic paste containing a noble metal is printed in a predetermined pattern on the surface of the solid electrolytic body. Then, heat treatment is performed to remove the binder and decompose the noble metal containing organometal, thereby forming the noble metallic nucleus by the noble metal depositing on the surface.
The organometallic paste may contain a di-benzylidene platinum. The noble metal may be Pt, Pd, Au, or Rh.
The reference electrode may be formed by chemical plating, conductive paste printing, sputtering or evaporation.
Furthermore, the present invention has an object to provide an oxygen sensing element excellent in response and in thermal durability.
To accomplish this and related objects, another aspect of the present invention provides an oxygen sensing element comprising a solid electrolytic body, a reference gas chamber provided in the solid electrolytic body, a sensing electrode provided on an outer surface of the solid electrolytic body, a reference electrode provided on an inner surface of the solid electrolytic body which defines the reference gas chamber. A gas receiving surface region, exposed to measuring gas when the oxygen sensing element is operated, is provided on the outer surface of the oxygen sensing element so as to extend from a distal end of the oxygen sensing element to a position spaced by a distance L away from the distal end of the oxygen sensing element. The sensing electrode has a length L1 equal to or larger than 0.2L in a longitudinal direction of the oxygen sensing element. The sensing electrode is entirely located in a region extending from the distal end of the oxygen sensing element to a position spaced by a distance 0.8L away from the distal end of the oxygen sensing element. And, the sensing electrode has a thickness of 0.5xcx9c3.0 xcexcm.
According to this arrangement of the present invention, the sensing electrode has the length L1 equal to or larger than 0.2L in the longitudinal direction of the oxygen sensing element. The sensing electrode is entirely located in the region extending from the distal end of the oxygen sensing element to the position spaced by the distance 0.8L away from the distal end of the oxygen sensing element. And, the sensing electrode has the thickness of 0.5xcx9c3.0 xcexcm. The distal end of the oxygen sensing element is an end portion protruding toward the measuring gas (refer to FIG. 19).
If the length L1 is less than 0.2L, the sensing electrode will shrink due to high-temperature heat and may cause the breaking. The sensor output and the response will be lowered.
A preferable upper limit of the length L1 is 0.8L. If the length L1 exceeds 0.8L, the sensor response may deteriorate.
If the thickness of the sensing electrode is less than 0.5 xcexcm, the sensing electrode will shrink due to high-temperature heat and may cause the breaking. If the thickness is larger than 3.0 xcexcm, the oxygen gas will not diffuse and penetrate well in the sensing electrode. Thus, the sensor response will be worsened.
According to the oxygen sensing element of the second aspect of the present invention, the length L1 of the sensing electrode is equal to or larger than 0.2L. This arrangement provides the sensing electrode having a sufficient area to prevent undesirable thermal shrinkage even when it is subjected to high-temperature gas for a long time. The breaking of the sensing electrode can be prevented. Thus, it becomes possible to provide an oxygen sensing element having excellent thermal durability.
The sensing electrode is entirely located in the region extending from the distal end of the oxygen sensing element to the position spaced by the distance 0.8L away from the distal end of the oxygen sensing element, when xe2x80x9cLxe2x80x9d represents the length of the gas receiving surface region where the solid electrolytic body is exposed to the sensing gas.
In general, the oxygen sensing element is assembled in an oxygen sensor. The oxygen sensor has a portion exposed to the measuring gas and a portion exposed to the reference gas. A metallic packing is provided to seal the boundary between them when the oxygen sensing element is installed. The metallic packing is adjacent to the edge of the gas receiving surface region on the oxygen sensing element, and prevents the measuring gas from advancing beyond this edge portion.
The measuring gas flows at a reduced speed in a region exceeding the 0.8L position due to the presence of the metal packing. If the sensing electrode is provided in this region, the sensor output will deteriorate.
Accordingly, it becomes possible to obtain an oxygen sensing element having satisfactory response by providing the sensing electrode in the region not exceeding the 0.8L position. In this case, the 0.8L position is included in the desirable region for the sensing electrode.
Furthermore, according to the present invention, the sensing electrode has the thickness of 0.5xcx9c3.0 xcexcm. This arrangement makes it possible to allow the measuring gas to diffuse and penetrate well in the sensing electrode. Thus, it becomes possible to obtain an oxygen sensing element having excellent response.
According to the above-described arrangement of the present invention, it becomes possible to provide an oxygen sensing element excellent in response and thermal durability.
The oxygen sensing element of the present invention is applicable to a oxygen concentration cell type oxygen sensor or a limiting-current type oxygen sensor. The sensing electrode may be an electrode formed at the distal end region of the oxygen sensing element (refer to FIG. 19) or, alternatively, can be a ring electrode formed along an outer surface of the solid electrolytic body except the distal end (refer to FIG. 30). Furthermore, the ring electrode can be replaced by a partly provided electrode (refer to FIGS. 31A and 31B).
The sensing electrode and the reference electrode are connected to terminal portions via lead portions to transmit sensing and reference signals to the outside.
Each electrode and associated lead and terminal portions can be fabricated integrally. The reference electrode and associated lead and terminal portions can be fabricated by chemical plating, paste printing, sputtering, or evaporation. The sensing electrode and associated lead and terminal portions can be fabricated in the same manner by using the same method.
It is preferable that the sensing electrode is a noble metallic electrode including at least one noble metal having catalytic activity, for example, selected from the group consisting of Pt, Pd, Au and Rh.
It is preferable that the lead portion of the sensing electrode and the lead portion of the reference electrode are not in an opposed relationship (refer to FIG. 20B). With this arrangement, it becomes possible to prevent a low-temperature lead portion from giving an adverse influence to the sensor output, thereby improving the sensor response.
It is also preferable that the gas receiving surface region is covered by a single layer or a plurality of layers so that the gas receiving surface region is indirectly exposed to the measuring gas.
Furthermore, it is preferable that the oxygen sensing element has the heater which comprises the heat generating portion accommodating the resistor element generating heat in response to supplied electric power. The sensing electrode is positioned at the position opposing to at least the central position of the heat generating portion in the longitudinal direction of the oxygen sensing element. And, the heat generating portion has the length L2 in the longitudinal direction of the oxygen sensing element so as to satisfy the relationship 1.0xe2x89xa6L1/L2xe2x89xa64.0.
As understood from later-described FIG. 23, the central position of the heat generating portion is a highest temperature portion. Disposing the sensing electrode at the position opposing to the central position of the heater is effective to increase the heating efficiency and the response of the heater.
When the heat generating portion satisfied the relationship 1.0xe2x89xa6L1/L2xe2x89xa64.0, it becomes possible to provide an oxygen sensing element excellent in response and thermal durability.
If the ratio L1/L2 is smaller than 1.0, the sensing electrode may shrink when subjected to high-temperature environment heated by the heater.
On the other hand, if the ratio L1/L2 is larger than 4.0, the temperature distribution in the longitudinal direction of the sensing electrode will have a large temperature difference due to presence of a low-temperature region. The low-temperature region will give adverse influence to the sensor output and deteriorate the response.
The heater may have a rod body, a flat platelike body, or the like.
Furthermore, it is preferable that the heat generating portion has the length L2 in the range of 3xcx9c12 mm.
With this arrangement, the generated heat can be effectively transmitted to the distal end region of the oxygen sensing element. When the oxygen sensing element is used to detect the oxygen concentration in the exhaust passage of an automotive vehicle, a dead time of the oxygen sensor can be shortened. In this case, the dead time is a period of time required for the activation of the oxygen sensing element until the oxygen sensing element operates properly after the engine is started up.
If the length L2 is less than 3 mm, the resistor element will be short correspondingly. Accordingly, the resistor element will have an insufficient resistance value. Generated heat will be unsatisfactory.
On the other hand, if the length L2 is larger than 12 mm, it takes a long time to increase the temperature of the heater. In other words, activation time is long. The activation time is a period of time required for the oxygen sensing element to reach a predetermined activation temperature from an ordinary temperature. The oxygen sensing element can operate properly only when the temperature exceeds the activation temperature.
Furthermore, it is preferable that the length L of the gas receiving surface region is in the range of 15xcx9c30 mm. With this arrangement, it becomes possible to reduce the temperatures of neighboring metallic parts. This is advantageous when the oxygen sensor is installed in an automotive vehicle.
As described above, the oxygen sensing element is used as a component of an oxygen sensor. The inside space of the oxygen sensor is separated into a portion where the measuring gas flows and a portion where air serving as the reference gas flows. The boundary between two portions is sealed. The sealed portion is adjacent to the edge of the gas receiving surface region.
When the length L of the gas receiving surface region is less than 15 mm, the sealed portion is positioned closely to the heat generating portion of the heater. The temperature of the sealed portion will be increased to a higher value.
In general, the seal of the oxygen sensing element is an assembly of metallic members which are elastically deformable. Thus, there is the possibility that the sealed portion may deteriorate when the environmental temperature of the sealed portion exceeds the durable limit of the metallic members. The measuring gas will be mixed with the reference gas, rendering the detection of the oxygen concentration inaccurate.
On the other hand, if the length L is larger than 30 mm, enlarged covers will be required for covering the oxygen sensing element (refer to FIG. 23). The large-sized oxygen sensor is not desirable when an installation space is limited.
Furthermore, it is preferable that the sensing electrode is fabricated by chemical plating.
The sensing electrode fabricated by the chemical plating has excellent response. According to the chemical plating, the plating film is sintered at a low temperature. This is effective to form an electrode having a high surface energy and excellent catalytic activity.
In general, the electrode fabricated by chemical plating has numerous fine pores which improve the diffusibility of oxygen gas.
It is preferable to form a noble metallic nucleus of a predetermined pattern on the outer surface of the solid electrolytic element prior to the chemical plating. The noble metallic nucleus is formed in the following manner. An organometallic paste containing a noble metal is printed in a predetermined pattern on the surface of the solid electrolytic body. Subsequently, heat treatment is performed to remove the binder and decompose the noble metal containing organometal, thereby forming the noble metallic nucleus by the noble metal depositing on the surface.
Then, the chemical plating is applied on the solid electrolytic body to form an electrode having the same pattern as that of the noble metallic nucleus. According to this method, a complicated sensing electrode can be easily formed.
Furthermore, it is preferable that the reference electrode and the sensing electrode are in an opposed relationship via the solid electrolytic body. With this arrangement, it become possible to prevent the electrode containing expensive noble metals from being unnecessarily widened, thereby reducing the manufacturing cost.
Furthermore, it is preferable that the solid electrolytic body is a cup-shaped body having one end closed and having an inner space serving as the reference gas chamber, and the heater is accommodated in the reference gas chamber.
The present invention can be applied to a so-called cup-shaped oxygen sensing element.
Furthermore, it is preferable that a clearance of 0.05xcx9c1.0 mm is provided between the heater and the inner surface of the oxygen sensing element at the longitudinal position corresponding to the sensing electrode.
With this arrangement, the solid electrolytic body can be effectively heated by the heater.
If the clearance is larger than 1.0 mm, generated heat will not be effectively transmitted to the solid electrolytic body due to convection caused in the widened space between the solid electrolytic body and the heater.
On the other hand, if the clearance is less than 0.05 mm, the diffusibility of the oxygen gas will be worsened. The sensor output will decrease due to lack of oxygen gas.
Furthermore, it is preferable that the oxygen sensing element is a multilayered sensing element, and the heater and the solid electrolytic body are accumulated layers of the multilayered sensing element.
The present invention can be applied to the multilayered oxygen sensing element which comprises platelike integrated layers of the solid electrolytic body and the heater layers.